Commercial airliners fly in what is known as the transonic regime at Mach numbers between 0.8 and 1.0. While the airplane itself never exceeds the speed of sound, that doesn’t mean that there aren’t localized regions where air flows over the airplane at speeds above Mach 1. In fact, it’s actually possible sometimes to see shock waves on the top of airliner’s wings with nothing more than your eyes. The animations above show shock waves sitting about 50-60% of the way down the wing’s chord on a Boeing 737 (top) and Airbus A-320 (bottom). The shock wave looks like an unsteady visual aberration sitting a little ways forward of the wing’s control surfaces.
The wings themselves are shaped so that these little shock waves are relatively stationary and remain upstream of the flaps pilots use for control. Otherwise, the sharp pressure change across a shock wave sitting over a control surface could make moving that surface difficult. This was one of the challenges pilots first trying to break the sound barrier faced. (Image credits: R. Corman, source; agermannamedhans, source)